1. Field of the Invention
The invention concerns a method for determining the field strength of radio-frequency pulses which are emitted in a magnetic resonance measurement by an antenna of a magnetic resonance measuring device. Moreover, the invention concerns a magnetic resonance measuring device having corresponding arrangement for determining the field strength of emitted radio-frequency pulses
2. Description of the Prior Art
Magnetic resonance tomography (MRT), also known as nuclear spin tomography, has become a widespread technique for obtaining images inside the body of a live examination subject. In order to obtain an image using this technique, the body or the body part being examined of the subject must be exposed to a static basic magnetic field (usually known as the B0 field) which is as homogeneous as possible, the basic magnetic field being generated by a basic field magnet of the magnetic resonance measuring device. While the magnetic resonance images are being recorded, this basic magnetic field has fast-switched gradient fields superimposed on it for spatial encoding, which are generated by gradient coils. Moreover, using radio-frequency antennas, radio-frequency pulses with a defined field strength are radiated into the examination subject. The magnetic flux density of these radio-frequency pulses is normally designated as B1, or rather the pulse-shaped radio-frequency field is generally known as the B1 field for short. Using these radio-frequency pulses, the nuclear spins of the atoms in the examination subject are excited such that they are deflected by a so-called “excitation flip angle” α (hereafter the “flip angle” α) from their equilibrium position parallel to the basic magnetic field B0. The nuclear spins then precess around the direction of the basic magnetic field B0. The magnetic resonance signals generated in this manner are recorded by radio-frequency receiving antennas. The receiving antennas can be either the same antennas which were used to emit the radio-frequency pulses or separate receiving antennas. The magnetic resonance images of the examination subject are generated based on the received magnetic resonance signals. Each image point in the magnetic resonance image is assigned to a small body volume known as a “voxel” and each brightness or intensity value of the images points is linked to the signal amplitude of the magnetic resonance signal received from this voxel. The relationship between the resonantly radiated B1 field and the flip angle α thus attained is given by the following equation in the case of a rectangular pulse:α=γ·B1·τ  (1)where γ is the gyromagnetic ratio, which can be considered to be a fixed material constant for most nuclear spin studies, and τ is the influence duration of the radio-frequency pulse. The flip angle α attained through an emitted radio-frequency pulse and thus the strength of the magnetic resonance signals depends accordingly, besides on the duration of the pulse, also on the strength of the radiated B1 field. Fluctuations in the field strength of the excitation B1 field thus lead to undesired variations in the received magnetic resonance signal which can corrupt the measurement result.
In an unfavorable manner, however, the radio-frequency pulses exhibit particularly in case of high magnetic field strengths—which are necessary due to the required magnetic basic field B0 of currently up to 3 Tesla in an MRT apparatus—an inhomogeneous penetration behavior in conductive and dielectric media such as tissue. The result is that the B1 field can vary widely within the measurement volume. In order to be able to take into account these variations of the B1 field during the measurement, e.g., during an adjustment of the B1 field or during an evaluation of the received magnetic resonance signals, it would be very advantageous if the effect could be determined quantitatively. For this purpose, a number of different techniques already exist which, however, are associated with diverse disadvantages in an unfavorable manner.
In one technique, a series of spin echo images are recorded. Initially a first excitation pulse is emitted which produces a flip angle α, and subsequently a further excitation pulse which produces a flip angle 2·α. Afterwards, the “echo signal” is measured. A classic example of such a spin echo recording is the emission of a 90° pulse (i.e., α=90°) and a 180° pulse which follows after a certain time span. In order to obtain information about the field strength at the different locations within a measurement volume, a number of series of such spin echo images are measured with different flip angles α. Since it is known that the dependency of the amplitude of the magnetic resonance signal on the angle a should be proportional to sin3α, by carrying out a corresponding fitting of curves which correspond to the nominal distribution to the measured distribution, the actually attained flip angle α and according to equation (1) also the actual B1 field can be determined for each image pixel. The disadvantage of such measurements is that the technique can be performed only in layers or slices, i.e. only a certain slice thickness of the volume is excited selectively through suitable switching of the gradient fields during the emission of the pulse. This is associated with a very long measurement time of approx. 10 minutes and, due to the layer selection, there is an additional flip angle distribution along the layer normals which results in a corresponding measurement error.
In another technique known hereafter as the “RF field technique”, a volume-selective excitation is first performed with a large flip angle α. “Large flip angle” is understood to mean flip angles of approximately 720° and greater. Then, a slice-selective spin echo refocusing takes place. In the images measured in this manner, stripe patterns are exhibited, all excitations which attain a certain flip angle α or an arbitrary multiple of this flip angle α exhibiting the same signal intensity. In other words, locations at which a flip angle of α=180° is attained are displayed identically to locations having a 360° flip angle or 540° flip angle. This technique again has the disadvantage that only individual layers can be investigated and, particularly in case of in vivo measurements, i.e., measurements inside of the subject, only qualitative evaluations are possible.